Direct Utilization of Low-Enthalpy Geothermal Energy for Space Heating and Cooling inside Building

Direct Utilization of Low-EnthalpyGeothermal Energy for Space Heating and

Cooling inside Building

Andhika Priotomo Rahardjo and Irpan Friyadi

Faculty of Engineering, Universitas IndonesiaKampus UI, Depok, West Java 16424, Indonesia (+62 21 7867222)

ABSTRACT

Almost 60% of the world’s electricity is consumed inresidential and commercial buildings. At the national level,energy use in buildings typically accounts for 20-40% ofindividual country total final energy use, with the worldaverage being around 30%. In many developing andindustrialized country, escalation of energy demand can besolved by using locally available and sustainable low-enthalpygeothermal resources. Certain geological locations provide lowenthalpy resources that are best adapted for directutilization. These are the so-called plate boundaries whichtake the form of spreading zones or rift zones. These zoneswill remain the main areas of interest until the time when HotSedimentary Aquifer (HSA) technology might allow geothermalproduction virtually everywhere in the world. Consequently,the countries that are positioned within these zones maybecome more independent from fossil fuel import because of thepossibility of using indigenous energy. Direct-use ofgeothermal energy is the most versatile and common forms ofutilizing geothermal energy. The total installed capacity,reported through the end of 2014 for geothermal directutilization worldwide is 70,329 Mega Watt, a 45.0% increaseover World Geothermal Congress (WGC) 2010, growing at anannual compound rate of 7.7%. The installed capacity nowtotals 7,556 Mega Watt and the annual energy use is 88,222Tera Joule per year. In comparison, 88% of the total installedcapacity and 89% of the annual energy use is in districtheating (28 countries). While, the installed capacity of spacecooling is 360 Mega Watt and the annual energy use is 2,600Tera Joule per year. Space heating can be provided by means ofpumped wells, heat pump, or through the use of down-hole heatexchangers. A space heater is intended to heat a space

directly, unlike a central heating furnace or boiler whichdistributes heat to the house through a system of ducts orpipes. Geothermal absorption refrigeration units make use ofthe boiling temperature of a liquid depends on pressure; heatis transferred from the environment when a liquid boils, andthe result is space cooling.

Keyword: Direct Utilization, Low-Enthalpy Geothermal, Space Heating, Space Cooling, Refrigeration.

1. INTRODUCTIONIn many countries, climate change has received more

attention by policy makers than any other environmentalproblem. The emission of CO2, which is the principal greenhousegas in the atmosphere, sulfur oxides (SOx) and nitrogen oxides(NOx) are the common gases that produce from fossil fuel basedenergy. For contribution of global warming and climate changemitigation, national authorities have to consider the impactsof fossil fuel consumption within their national territories.

Geothermal energy is a clean energy resource, which couldsignificantly contribute to reduction of greenhouse and othergas emissions by replacing fossils fuel for energy. Geothermalresources can provide a stable supply of energy, in contrastto many alternative renewable resources, such as hydroelectricpower.

The geothermal resources of earth are huge. The part ofgeothermal energy stored at a depth of 3 kilometer isestimated to be 43,000,000 EJ corresponds to 1,194,444,444 TWh(Bijornsson et al. 1998). The potential of low enthalpygeothermal resources (<150oC) are widespread and occur atshallow depth than the high enthalpy geothermal resources(>150oC). But, these potentials receive little attention notonly in most of the developing country, but also in severalindustrialized countries.

Low enthalpy geothermal resources can be used for directutilization. There are many direct utilization of low enthalpygeothermal, for example are heat pumps, space heating, greenhouse heating, aquaculture pond heating, agriculture drying,industrial uses, bathing, swimming, space cooling, snow

melting, etc. For space heating and cooling, we can usegeothermal heat pump system and vertical loop to gather heatfrom low enthalpy geothermal reservoir. Direct utilization ofgeothermal energy in homes and commercial operations is muchless expensive and producing less gas emission than usingfossil fuel.

Figure 1. Comparison of worldwide direct-use geothermal energyin TJ/year from 1995, 2000, 2005, 2010 and 2015.

(Source: www.pangea.stanford.edu/ERE/db/WGC, accessed at 19th May 2015)

Geothermal energy potentials and total energy for direct use in different regional can be seen in this table below:

Table 1. Regional geothermal energy potential

(Source: Chandrasekharam. Low-Enthalpy Geothermal Resources for PowerGeneration. 2008)

Gawell et al.1999

Advanced

Bijornsson etal. 1998

Useful

Stefansson.1998

technologypotential data

accessibleresource base

Known Geothermal Potential

Direct Use Low-enthalpy direct use

Region TWh/year EJ EJ/year

North America 200 75555 >120

Latin America 354 100969 >240

Europe 97 105035 >370

Asia and Pacific

337 170007 >430

Africa 101 146936 >240

World 1089 598529 >1400

Figure 2. Counties with reported data on direct use ofgeothermal resources and other countries where geothermal

springs indicate geothermal activities.

(Source: Lund et al. 2005)

2.1 LOW ENTHALPY GEOTHERMAL

Geothermal energy is heat that is stored in the rock andfluid contained in the Earth’s crust (Geothermal ResourcesCouncil, 2011). The heat is generated by the natural decayover millions of years of radiogenic elements, includinguranium, thorium and potassium (Geoscience Australia andAustralian Bureau of Agricultural and Resource Economics,2010). This heat is constantly moving from the Earth’s core tothe surface and can therefore provide a sustainable energysource. Low temperature geothermal resources (withtemperatures typically below 150⁰C (Geothermal ResourcesCouncil, 2011) can be used directly (without conversion toelectrical energy) for the purpose of heating or cooling.Heating applications can include: agricultural purposes (forexample, greenhouse heating), industrial purposes (forexample, evaporation drying, sterilization and chemicalextraction), water desalination, bathing, aquaculture, andspace heating (KPMG, 2010).

Geothermal resources are classified based on theirreservoir temperatures alone (e.g. Muffler & Cataldi, 1978;Hochstein, 1990; Benderitter & Cormy, 1990; Haenel et al.,1988) or with reference to their specific exergy index toreflect their ability to do thermodynamic work (Lee, 2001). Inthis paper, the classic approach related to the geothermaltemperature is considered. According to Haenel et al. (1988),a low‐enthalpy resource corresponds to reservoir temperatureof less than 150°C. High‐enthalpy resources are present if thetemperature exceeds 150 °C (Chandrasekharam & Bundschuh,2008).

Until recently, geothermal exploitation was limited toconventional systems – in areas with high temperatures andwhere the fluid will transfer to the surface withoutadditional lift (International Energy Agency, 2011). Newtechnologies now permit the exploitation of deeper and coolerresources as found in hot sedimentary aquifers and enhancedgeothermal systems (KPMG, 2010). These technologies havesignificantly expanded the global geothermal resourcepotential.

Most of the low enthalpy geothermal systems are cyclicsystems with rain water as the main carrier of heat from the

deeper parts of the earth to the surface. Depending on thelocal geological and thermal regimes, the systems could besteam-dominated or liquid-dominated systems. To viable forexploitation, these systems should be accessible at reasonabledepths with sufficient geothermal fluids to sustain longproductivity. Low enthalpy geothermal resources occur also asgeopressured systems in large sedimentary basins that have notbeen exploited for commercial exploitation.

Figure 3. Geothermal gradient data and corresponding depths,where geothermal resources with a suitable temperature for

direct utilization of 80oC can be expected.

(Source: International Heat Flow Commission. 2010)

2.1.1 HOT SEDIMENTARY AQUIFERS

Hot sedimentary aquifers (HSA) are found in parts of theworld where there are deep (usually saline) aquifer systemswith higher than average geothermal gradients, and where thereis a thermal blanket, such as shale’s or coals, overlying theHSA. In addition, the aquifer needs permeability, via faultsof fractures, so that the hot water can be produced via deepwell. The temperature of the water in a HSA is typically up to

100oC. The advantage that HSA systems had over volcanicgeothermal system is that they are more widespread.

Hot sedimentary aquifer systems are similar toconventional geothermal systems. They contain naturallyoccurring reservoirs of hot water or steam. To use thesereservoirs it is necessary to drill both a production andinjection well. The key difference between hot sedimentaryaquifers and conventional systems is that hot sedimentaryaquifers involve drilling into hot sedimentary basins withtemperatures typically lower than conventional systems.Furthermore, they occur at greater depths than conventionalsystems. HSA systems are typically developed in naturallyoccurring porous sandstones containing water that is heated byeither crustal heat flow or proximate hot rocks.

Figure 4. Diagram showing the three different geothermalenergy types, volcanic hydrothermal, hot sedimentary aquifer,

and enhanced geothermal or hot rocks.

(Source: Australian Geothermal Energy Association. 2005)

Geothermal HSA modeling is a two component model, wherebyboth groundwater fluid flows is first described, and then heatflow is detailed. The mathematical equations describing bothphysical processes are very similar, consisting of fluxrelationships like Darcy‘s law and diffusive partialdifferential equations. Boundary conditions used in the twodisciplines are analogous. Model data requirements are doubledfor geothermal exploration, in that aquifer propertiesrelating to both fluid and heat transport must be described.

There are important differences between the geothermaland groundwater modeling efforts, however. Heat is transportedby both the aquifer fluid and the rock matrix, and heat iscreated by the surrounding rocks themselves. The scale ofmodeling is usually different, as groundwater focuses on near-surface fresh resources, while geothermal often targets deepbasin-scale supplies. Two dimensional vertically averagedmodeling common in shallow groundwater aquifers is replaced bya focus on changes in depth with geothermal regimes. Likewise,heat flow modeling usually considers boundary influences atdepth rather than surficial fluxes like groundwater recharge.Thermal aquifer properties are rarely measured, and mistakendata is a serious problem. Additional differences relate tothe range of conditions considered. In geothermal exploration,the constitutive properties are dependent upon temperature andpressure, so typical simplifying assumptions like hydraulicconductivity must be expanded to include matrix permeabilityand explicit descriptions of water viscosity and density.Finally, natural convection caused by the coupling of fluidand heat flow can show fluid and heat in complicated multi-dimensional ways.

2.2 HEAT PUMP

Basically, heat pump is nothing more than a refrigeratorthat can be reversed. The other refrigeration device (AC,refrigerator, freezer) moves heat from a space where it isn’twanted and discharged that heat somewhere else. The onlydifference between a geothermal heat pump and ordinaryrefrigerator is that heat pump are reversible and can provideeither heating or cooling to almost any space. Example, ifgeothermal heat pump is used for cooling/heating spaces suchas building, it is also used to heat swimming pool or forshower.

If the geothermal heat pumps are used in four seasoncountry, it’s very use full. For example, if in warm weatherthe heat pumps act as refrigerator. It removes heat from theroom being cooled building and deposits that unwanted heat tothe earth. In cool weather the heat pump as a its reversed,withdraws the heat from heat and transfer the heat to theroom.

The geothermal heat pump is usually packaged in a singlecabinet, which includes the compressor, loop to refrigerantheat exchanger and controls. Systems that distribute heatusing ducted air also contain the air handler, duct fan,filter, refrigerant to air heat exchanger, and condensateremoval system for air conditioning. For home installation,usually heat pumps are installed in a basement, attic or eventcloset.

Figure 5. Geothermal heat pumps(Source : www.welldrilingschool.com, accessed at May 21st, 2015)

The illustrated below show how the geothermal heat pumpsworks in order to space heating and cooling:

Figure 6. How a geothermal heat pump works.(Source : www.welldrilingschool.com, accessed at May 21st, 2015)

There are three basic components in geothermal heat pumpsystem,

Heating/ cooling delivery system: traditional ductwork /piping system to deliver heat throughout the building

Heat pump: mechanical unit that moves heat from theworking fluid then concentrates it and transfers heat tothe circulating air.

Ground heat exchanger: underground piping systems thatuses a working fluid to absorb or reject heat from theground.In heat pump system, used a vapor compression cycle to

transport heat from one location to another. Usually heat pumpuses ammonia-water or lithium bromide for refrigerant. Thetemperature from hot sedimentary aquifer is about 80-120oC andthis temperature is enough to evaporated the ammonia-waterfluid. In heating mode, the cycle starts as the cold liquidrefrigerant within the heat pump passes through a heatexchanger (evaporator) and absorbs heat from the fluidcirculated through the well connection. The refrigerant

evaporates into a gas as heat is absorbed from the water fromaquifer heat exchanger (evaporator). The gaseous refrigerantthen passes through a compressor where it is pressurized,raising its temperature. The hot gas then circulates trough arefrigerant to air heat exchanger where the heat is removedand sent through the air ducts. When the refrigerant loses theheat, it changes back into a liquid. The liquid refrigerantcools as it passes through an expansion valve and the processbegins again.

In cooling mode, the cycle starts as a hot gases andpasses through the heat exchanger. In this heat exchanger, thetemperature from the refrigerant may higher than water fromthe aquifer. Because of that, the heat is transferred from therefrigerant into aquifer. The refrigerant which lost theirheat then passes through an expansion valve. From thethermodynamics, we know that if a fluid is passes through anozzle the temperature will decrease. This refrigerant may becondensed to liquid cool and then through the evaporator. Thisevaporator will be installed in indoor. After passes thisevaporator, the refrigerant will evaporate again into gas.But, the condition of the refrigerant is actually inequilibrium liquid vapor. To make sure the refrigerant is invapor condition, the refrigerant is also passes the secondevaporator and then go to compressor. In this compressor, therefrigerant will have a pressure and its temperature willincrease again and the process will begin again.

The system that descripted above is a system with aforced air/ductwork distribution system, the most commongeothermal application. If the geothermal heat pump is used toheating a swimming pool or radiant floor heaters, the systemsmay have refrigerant to liquid or liquid to liquid heatexchangers instead of refrigerant to air. Moreover, thatsystem may also be equipped with a device called a superheater that can heat household water, which circulates intothe regular water heater tank.

After knew the systems at the surface of the aquifer, theanother question to answered is how the heat from aquifer canbe extract to generated the systems? Actually, there are manydesigns of connecting the aquifer with the systems at thesurface. But, if categorized we have two systems for extractthe heat from the geothermal systems there are close loop and

open loop. Many factors affect the design of the loop sectionof the earth connection of geothermal system. The designfactors to decide the loop for connecting the aquifer are :

Geologic condition (the thermal and hydrauliccharacteristics of the underground);

Technical parameters (length and type of ground heatexchanger, type and quality of grouting);

Other technical factors include the heating/coolingload, the type of space to be heated/cooled, and thesupply temperature from underground.

If we discussing about the advantage of each systems weknow that is the advantage of closed systems is theindependence from aquifers and water chemistry. This isbecause if we choose the closed systems, the circulated fluidis located inside the pipe which is flow down into theaquifer/wells and sucked again to the surface systems. Inother hand, the advantage of open systems is the higher heattransfer capacity of the wells or aquifer compared to aborehole.

In this paper, we describe that the source that will beused is hot sedimentary aquifer. This source hasn’t a waterjust a hot dry rock which can produce heat. Sure, in thiscased the systems that suitable to the conditions is closeloop. This is according to the condition of the reservoirwhich hasn’t water which can be used for the process.

There is many type configuration of down hole heatexchanger, which are horizontal closed loop, vertical closedloop and slinky coils closed loop. The horizontal closed loopis usually the most cost effective when adequate yard space isavailable and trenches are easy to dig. Using trenches, orbackhoes digging trenches three to six feet below the ground,the lay a series of parallel plastic pipes. The slinky coilsclosed loop is used for reduce the heat exchanger per foottrench requirements but require more pipe per ton of capacity.This pipe is coiled like a slinky, overlapped and laid in atrench.

(a) (b)Figure 7. (a) Horizontal Closed Loop (b) Slinky Coils Close

Loop(Source : www.welldrilingschool.com, accessed at May 21st, 2015)

For hot sedimentary aquifers, the configuration of downhole heat exchanger can used vertical closed loop. Theadvantage of this type of loops is no need a big yard space,when surface rocks make digging impractical, or when you wantto disrupt the landscape as little as possible. Vertical holesare bored in the ground and single or multiple loops of pipewith a U-bend at the bottom is/are inserted before the hole isbackfilled. Each vertical pipe is then connected to ahorizontal underground pipe that carries fluid in a closedsystem to and from the indoor exchange unit. This type ofconfiguration has a weakness which is more expensive toinstall, but require less piping than horizontal loops becausethe aquifer temperature is more stable farther below thesurface. An important factor for design this typeconfiguration is the spacing between boreholes. According torule of thumb, the distance of boreholes should be 15-20 feetapart to avoid having the thermal conductivity of boreholesconflicting with each other. Vertical ground loops typicallyrequire 150-300 square feet of land area per systems ton ofheating/cooling capacity. The usual range of boreholediameters is four inches for ¾ inch piping, five inches for 1inch piping and six inches for 1 ¼ inch piping. The diameteris controlled by the radius of the U-joint needed for the pipediameter. This pipe is made from HDPE materials.

Figure 8. Vertical Close Loop(Source : www.welldrilingschool.com, accessed at May 21st, 2015)

2.2.1 ADVANTAGES USING HEAT PUMP

Energy efficiency Heating efficiency is expressed ad a coefficient of

performance (COP); the higher of COP value show the moreefficiency of systems. Heat pump systems for building may havea COP 4,3 which means that for every unit of energy used topower the system, 3.4 units are put back into thehome/building as heat / cooling. If compares with othersystems such as gas furnace is 0.92. the cooling efficiency ismeasured as an energy efficiency ratio (EER); the higher ofEER show the more efficiency systems. COP and EER aredependent on a number of factors. A heat pump can save up to30-40% of the electricity typically used for heating orcooling in homes. In mild and moderate climates, heat pumpscan provide two to three times more heating than theequivalent amount they consume in electricity.

Environmental benefits

Because the efficiency of heat pumps systems, it uses alot less energy to maintain comfortable indoor temperature.According to EPA, geothermal heat pump can reduce energyconsumption up to 44% compared to air source heat pumps and upto 72% compared to electric resistance heating with standardair-conditioning equipment.

Cost effectiveGeothermal heat pumps save money in operating and

maintenance costs. While the initial investment for purchase aheat pump for building is often higher than that comparablegas-fired furnace and central air-conditioning system, it ismore efficient, in terms saving money every month. An averagegeothermal heat pump system costs about $7,500 (plusinstallation and drilling cost). Comparable ASHP systems withair conditioning would cost about $4,000, but the energy costshould easily equate to the extra cost of installing ageothermal heat pump.

Long durability Because use fewer mechanical components and because those

components are sheltered from the elements, leaves, dirt andpossible vandalism, geothermal heat pumps are durable andhighly reliable. The underground piping used in the systemoften has 25 to 50 year warranties and the geothermal heatpumps typically last 20 years or more.

3. CONCLUSION

Challenging with global warming problem, every countries needto find solution of using renewable energy to reducing the useof power generation. Low enthalpy geothermal energy such asHSA is a clean energy with almost zero emission, widespreadaround the world, and can be used for direct utilization forspace heating and cooling. To gather the geothermal energy,vertical close loop is used so it can be transferred to theheat pump. The heat pump general function is to moves heatfrom a space where it isn’t wanted and discharged that heatsomewhere else. There are three basic components in geothermalheat pump system, heating/ cooling delivery system heat pump,and ground heat exchanger. The advantages of using heat pumpare energy efficiency, cost effective, and long durability.

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